Changes in Inorganic Fine Particulate Matter Sensitivities to

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Policy Analysis pubs.acs.org/est

Changes in Inorganic Fine Particulate Matter Sensitivities to Precursors Due to Large-Scale US Emissions Reductions Jareth Holt,*,† Noelle E. Selin,†,‡ and Susan Solomon† †

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 54-1711, Cambridge, Massachusetts 02139, United States ‡ Engineering Systems Division, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: We examined the impact of large US emissions changes, similar to those estimated to have occurred between 2005 and 2012 (high and low emissions cases, respectively), on inorganic PM2.5 sensitivities to further NOx, SO2, and NH3 emissions reductions using the chemical transport model GEOS-Chem. Sensitivities to SO2 emissions are larger yearround and across the US in the low emissions case than the high emissions case due to more aqueous-phase SO2 oxidation. Sensitivities to winter NOx emissions are larger in the low emissions case, more than 2× those of the high emissions case in parts of the northern Midwest. Sensitivities to NH3 emissions are smaller (∼40%) in the low emissions case, year-round, and across the US. Differences in NOx and NH3 sensitivities indicate an altered atmospheric acidity. Larger sensitivities to SO2 and NOx in the low emissions case imply that reducing these emissions may improve air quality more now than they would have in 2005; conversely, NH3 reductions may not improve air quality as much as previously assumed.

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condense.3,9 However, multiple reactions oxidize SO2 into SO42−, and each reaction responds differently to NOx and hydrocarbon concentrations.10,11 PM2.5 sensitivities have been estimated using a variety of approaches, including finite differences,12,13 direct decomposition,14−17 and adjoint modeling.18,19 Sensitivity estimates calculate derivatives based around atmospheric conditions associated with particular emissions. Extrapolating these estimates to other levels of emissions is associated with some degree of error. Zhang et al.17 show that a linear extrapolation underestimates the PM2.5 response to a 50% decrease in NOx emissions by 15%, averaged over the US, with local underestimates up to 50%. Higher-order sensitivity analysis can more accurately predict responses beyond the linear range,14 but these approaches are computationally demanding. Studies of health and economic impacts of emissions often apply estimates of PM2.5 sensitivities to parametrize how air quality responds to emissions. Muller and co-workers12,20,21 use the integrated assessment model APEEP to calculate marginal damages (in US dollars per ton) by increasing emissions from one source by one ton and tracing impacts on human health, agriculture, and other vulnerable structures. The base case in APEEP uses the EPA’s 2002 National Emissions Inventory (NEI02), but Muller21 implements the 2005 inventory

INTRODUCTION Fine particulate matter (PM2.5) adversely affects cardiovascular and respiratory functioning1 and is a key focus of air quality policies such as the National Ambient Air Quality Standards (NAAQS). Designing effective policies requires knowledge of how PM2.5 responds to changes in its precursors−its sensitivity to emissions. The precursors of inorganic PM2.5 are nitrogen oxides (NOx), sulfur dioxide (SO2), and ammonia (NH3). US NOx and SO2 emissions decreased by 42% and 62%, respectively, between 2005 and 2012, while NH3 emissions remained approximately constant.2 These emissions changes are potentially large enough to change the sensitivity of PM2.5 to future emissions reductions and hence change the expected benefits of air quality policies. We analyze where and to what extent large changes in precursor emissions, similar to those that occurred between 2005 and 2012, alter US PM 2.5 sensitivities to further emissions reductions. Several chemical mechanisms affect PM2.5 concentrations. Nitric acid (HNO3) is formed from NOx, and the fraction of NH3 and HNO3 in particulates (the partitioning of these species) depends on ambient temperature, humidity, and acidity. NH3 is the primary basic species, forming ammonium (NH4+) in particles to neutralize acidic nitrate (NO3−) and sulfate (SO42−, formed from SO2). PM2.5 sensitivities to NH3 emissions are large, and it has been argued that reducing NH3 is a cost-effective strategy to reduce PM2.5.3−8 Sulfate is not volatile like NH3 and HNO3, but it influences the ambient acidity. PM2.5 concentrations can even increase as SO42− concentrations decrease, by allowing more HNO3 to © 2015 American Chemical Society

Received: Revised: Accepted: Published: 4834

January 1, 2015 March 25, 2015 March 27, 2015 March 27, 2015 DOI: 10.1021/acs.est.5b00008 Environ. Sci. Technol. 2015, 49, 4834−4841

Policy Analysis

Environmental Science & Technology (NEI05). Fann, Baker, and Fulcher22,23 use the CAMx Particle Source Apportionment Technology to attribute PM2.5 concentrations to emissions from each economic sector. Their simulations use NEI05 and projections for 2016 based on proposed air quality rules. Similarly, Fann, Fulcher, and Hubbell24 use NEI02 with projections for 2015 as the emissions inventory for EPA’s Response Surface Model of air quality.25 In these studies, the projections based on thenproposed rules exhibit 30% decreases in national NOx and SO2 emissions between 2001 and 2010.25 These emissions actually decreased by 34% and 53%, respectively, and NH3 emissions increased by 17%. Whether sensitivities calculated using older (2002 and 2005) emissions, or even past estimates of current emissions, are sufficiently accurate for health and economic impact assessment depends on the nonlinear response of PM2.5. We evaluate the influence of large NOx and SO2 emissions changes (comparable to those that occurred in the US between 2005 and 2012) on PM2.5 sensitivities and identify the most important nonlinear processes. We find that lower NOx emissions lead to higher SO2 sensitivities across the US and to higher NOx sensitivities in winter in cold, humid regions such as the northern Midwest. Lower NOx and SO2 emissions also yield smaller sensitivities to NH3. Our results suggest that the benefits of NOx reductions could be much larger now that emissions are lower, especially in winter (when NH3 emissions were thought to be dominant). We also show that SO2 controls are still effective, despite >60% reductions nationally. The potential changes identified highlight the need to review the sensitivities used in health, economic, and policy studies and consider a multipollutant approach to air quality policy.

seen in GEOS-Chem.28,38,39 These features improve simulated NOx concentrations (especially the daytime-nighttime difference) and reduce the high NO3− bias noted previously.28,29,38 Emissions. We use anthropogenic emissions from the EDGAR and RETRO global inventories plus several regional inventories (over e.g., China, Europe).40 The US inventory is EPA’s National Emissions Inventory for 2005 (NEI05). NEI05 provides emissions of NOx, SO2, NH3, several hydrocarbon species, and primary PM. We seasonally adjust NH3 emissions from NEI05 following Zhang et al.,38 decreasing winter emissions to better match observations, consistent with process-based inventories.41 We created two groups of simulations, based around a high and low emissions case, to test the influence of large-scale emissions reductions on PM2.5 sensitivity. The national, annual total emissions of NOx, SO2, and NH3 as reported to the US EPA for 2005 and 20122 exhibit a 42% decrease in NOx, a 62% decrease in SO2, and a 1% increase in NH3 emissions. We used these ratios to scale the high emissions case (using NEI05) to the low emissions case. Table 1 shows the resulting total Table 1. Anthropogenic Emissions (in kilotonnes = 106 kg) of Inorganic PM2.5 Precursor Emissions over the North American Domain in the Base Simulations of Each Case (High and Low Emissions) and the Changes in Emissions ΔE for Sensitivity Calculations E (high)

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NOx SO2 NH3

METHODOLOGY Chemical Transport Model. We use the GEOS-Chem chemical transport model v9-0226,27 (http://geos-chem.org/). GEOS-Chem has previously been evaluated against measurements of ozone and hydrocarbon concentrations26 and inorganic PM2.5 component concentrations.7,28,29 It has been used to study US air quality7,8 and climate change impacts on PM2.5 formation.30−32 GEOS-Chem simulates ozone-NOx-hydrocarbon-aerosol chemistry27 coupled to inorganic aerosol thermodynamics, which determines the partitioning of NH3 and HNO3. The thermodynamic module is ISORROPIA II,33 incorporated into GEOS-Chem by Pye et al.30 We use nested-grid simulations34,35 with at 0.5° × 0.67° resolution (55 km × 57 km at 40° N) over North America (10°−70° N, 140°−40° W) and 2° × 2.5° resolution (222 km × 213 km) elsewhere. The chemical mechanism has a temporal resolution of 20 min and a vertical grid of 47 layers extending to 80 km, with 30 layers in the lowest 11 km and 14 layers in the lowest 2 km. Our simulations are driven by GEOS5 meteorology from the NASA Global Modeling and Assimilation Office (GMAO). Each simulation uses meteorology for January or July of 2005, representing winter and summer conditions. GEOS-Chem v9-02 has three new features relevant to our simulations. First, it includes soil NOx emissions that respond dynamically to meteorology and nitrogen deposition.36 Second, it limits planetary boundary heights from falling lower than a friction velocity-based minimum. This corrects abnormally low nighttime boundary layers in GEOS5 meteorology compared to observations37 and improves the diurnal variability in simulated chemistry.28 Third, it reduces the rate of production of NO3− from N2O5 hydrolysis, reducing some of the high nitrate bias

ΔE

E (low)

Jan

Jul

Jan

Jul

Jan

Jul

1343.3 849.4 169.9

1248.9 834.2 548.6

909.6 491.0 170.7

855.3 484.5 552.3

206.5 115.6 15.9

187.5 112.8 73.8

anthropogenic emissions over the nested-grid domain. CO, VOC, and primary PM2.5 emissions in the US also changed by −16%, +1%, and +12% between 2005 and 2012. Primary PM2.5 emissions have a direct effect on total PM2.5 levels, so sensitivities to primary emissions stay constant. Changes to VOC and CO emissions would have effects on both organic and inorganic PM2.5 components. Since organic PM2.5 is not included in our study, we did not change VOC or CO emissions, but we do discuss the potential oxidative impact of CO emissions changes. While our scaling approach matches changes in total emissions, the spatial pattern of sources may have changed as well. Reported NO2 column densities over major cities in the US are between 24% and 48% lower in 2012 than in 2005,42 but NO2 concentrations in the four quadrants of the US are ∼37−40% lower in 2011 than in 2005.43 Russell, Valin, and Cohen44 estimate that NOx emissions from major power plants decreased by 26% between 2005 and 2011, while mobile emissions decreased by 34%. Fioletov et al.45 find that SO2 concentrations over major US power plants are consistently ∼40% lower in 2008−2010 than in 2005−2007. This previous work suggests that the total emissions decreases are distributed broadly across the country and across sectors. Hence, our scaling approach approximates the actual emissions changes. We calculate PM2.5 sensitivities as the finite difference in PM2.5 concentrations between simulations with emissions slightly increased and decreased around the baseline. Specifically, sensitivity is computed as 4835

DOI: 10.1021/acs.est.5b00008 Environ. Sci. Technol. 2015, 49, 4834−4841

Policy Analysis

Environmental Science & Technology S(c ) =

PM 2.5(E(c) + ΔE) − PM 2.5(E(c) + ΔE) 2 × ΔE

simple scaling to lower model SO42− would bring the NO3− NMB down to 97%. Modeled NO3− is thus better simulated in the region where its behavior is most important to our analysis, detailed below. PM2.5 Concentrations. Figure 1 shows the total inorganic PM2.5 concentrations from the high and low emissions cases. Figure S1 shows the components individually.

(1)

where c is the case (high or low emissions) with national-total emissions mass E(c); ΔE is the mass change in emissions; and PM2.5(E) is the PM2.5 concentration in the simulation with emissions E. Emissions of other species are fixed at their baseline values for that case. The resulting sensitivities have units of ng m−3 kt−1, where kt denotes 1000 t of emissions. The mass changes in emissions for each species and each season are in Table 1. Calculating sensitivities using mass changes helps clarify the mechanisms that contribute most to sensitivity changes. Since PM2.5 is the aggregate of multiple species, normalized (%-based) sensitivities can change between the emissions cases even if oxidation, deposition, and transport processes remain constant. We also use a centered finite difference, as opposed to a one-sided difference that may be more reflective of the effects of a regulation (i.e., a decrease in emissions). This allows our results to be comparable to the adjoint and direct decoupled methods of calculating sensitivities, which also produce centered derivatives.

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Figure 1. Spatial maps of the modeled surface concentrations of inorganic PM2.5. Columns show the high and low emissions cases; rows show January and July averages.

RESULTS Model Evaluation. We evaluated model performance by comparing inorganic PM2.5 component concentrations in our high emissions case (using NEI05 emissions) to measurements in January and July of 2005 from two monitoring networks: the Interagency Monitoring of Protected Visual Environments (IMPROVE) network46 and the EPA Air Quality System (AQS47). Here we report the coefficient of determination (squared correlation, r2) and the normalized mean bias (NMB, model mean over observed mean minus one). The statistics use measured and modeled concentrations paired in both space and time. A more detailed evaluation is provided in the SI (Tables S1 and S2 and Figures S3 and S4). Our simulation correlates reasonably well (r2 > 30%) with several measurements: January IMPROVE measurements of all species; both IMPROVE and AQS measurements of July SO42−; and January AQS measurements of NH4+. Modeled SO42− is unbiased in January (NMB < 5%) but slightly low in July (NMB ∼ −15%) compared to either set of measurements. Modeled January NH4+ and NO3− concentrations are biased high (NMB = 86%, 89% respectively, compared to AQS; NMB = 51%, 134% compared to IMPROVE) consistent with previous GEOS-Chem analyses.28,29,38 Simon, Baker, and Phillips48 compare published performance statistics from a range of chemical transport models (not including GEOS-Chem). They find that modeled SO42− is unbiased (NMB < 15%), whereas NO3− is biased high in winter (NMB 0% to 50%) and low in summer (−15% to −75%). Squared correlations for SO42− and NH4+ are between 25% and 60%, compared to 10%−45% for NO3−. Comparing their results to our statistics indicates that GEOS-Chem has a higher bias in winter NO3− than is typical but otherwise performs similar to other models. Within the northern Midwest − the same region as used for the thermodynamic analysis in Figure 3 − the NMB of modeled NO3− compared to IMPROVE measurements is 109% and r2 = 42%, showing that GEOS-Chem estimates NO3− in this area better than in the national average (NMB = 134%, r2 = 39%). While sulfate biases are generally smaller than nitrate, modeled SO42− is low in this area (NMB = −48%), and a

January PM2.5 peaks in the northern Midwest and is elevated over the eastern US. Northern Midwest PM2.5 is primarily composed of NH4+ and NO3− with low SO42−. National average NO3−, NH4+, and PM2.5 concentrations in the low emissions case are 7.7%, 9.5%, and 11.6% lower than in the high emissions case, respectively. However, these decreases are not uniform across the US. The area around Kentucky, Ohio, and Virginia shows higher aerosol NO3− in the low emissions case than in the high emissions case. Higher NO3− is offset by lower SO42−, so total PM2.5 concentrations are 10% of the (local) sensitivity to NH3. 4836

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Figure 2. Sensitivities of PM2.5 concentrations to emissions of the precursors NOx, SO2, and NH3, in units of ng m−3 of PM2.5 per thousand metric tons (kt) of emissions. The top and bottom panels show sensitivities in January and July, respectively. The columns show the high emissions case, the low emissions case, and their difference.

We find slight (100 km) show lower PM2.5 concentrations, but the relative changes are much larger for other components of PM2.5 than nitrate, sulfate, and ammonia. Our 55 km resolution is therefore sufficient for studying the regional response of inorganic PM2.5 to large, nation-wide changes in emissions, and the computational efficiency of the lower resolution allowed us to explore sensitivities (requiring several simulations for each case). The change in sensitivity to NH3 emissions has several implications. First, NH3 emissions controls have been identified as a potentially cost-effective way to improve air quality.5 We do not analyze the costs of emissions controls, though the costs of SO2 and NOx controls have likely changed from the redistribution of sources, but the impacts of NH3 emissions controls could be much smaller than previously estimated. Second, previous studies comparing modeled and measured PM2.5 in the US7,29,38,56 have highlighted our generally poor understanding of the magnitude and seasonality of NH3 emissions. Decreased sensitivity to NH3 would limit the adverse effects of inaccurate emissions on model performance. An alternative approach to our sensitivity analysis is to vary emissions based on economic sector (e.g., Caiazzo et al.13). However, simultaneous emissions changes in multiple sectors will not have the impact on PM2.5 expected from changes in each sector individually. The changes in sensitivities presented here will help identify which sectors could be expected to have strong interactions. For example, broad agricultural NH3 and NOx emissions can determine the neutralizing and oxidizing capacity of the background atmosphere and hence the impact of given coal SO2 emissions on PM2.5. Through this analysis, we find that lower NOx and SO2 emissions lead to larger sensitivity to SO2; smaller sensitivity to NH3; and larger sensitivity to winter NOx emissions in the US. These interactions provide new avenues for effective air quality regulations and emphasize the need to consider multiple pollutants simultaneously.

high emissions case and OH concentrations are 10.7% lower, supporting the link between NOx emissions and SO2 oxidation. We also investigated whether increased aqueous-phase oxidation would lead to faster SO2 and SO42− rainout due to more sulfur chemistry occurring within cloud droplets. The wet deposition rate (units s−1) is a measure of the speed of rainout (calculation details in the SI). Wet deposition rates are larger in the low emissions case than in the high emissions case by about 5%, compared to 50% larger rates of aqueous SO2 oxidation. In addition, the differences in PM2.5 sensitivities to SO2 emissions between the high and low emissions cases are spatially correlated (r2 = 57%) with the fraction of aqueous-phase oxidation. Thus, while faster rainout occurs under low NOx emissions, it cannot compensate for the increase in aqueous oxidation. We did not include the 16% decrease in CO emissions between 2005 and 2012 in our simulations. CO reacts with OH to form HO2 as the counterpart to NO + HO2 → NO2 + OH. Lower CO emissions would lead to a larger OH/HO2 ratio, less H2O2 production, and more gaseous SO2 oxidation. Thus, lower CO emissions could partially offset the shift to more aqueous-phase SO2 oxidation in our simulations. However, Duncan et al.50 suggest that much of the US is now in a NOxlimited ozone formation regime and hence that NOx exerts more control on HOx partitioning (and thus the SO2 oxidation pathway) than CO does.

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DISCUSSION Our study shows large differences in the sensitivities of PM2.5 concentrations to precursor emissions between two sets of simulations representing a 2005 baseline (high emissions) and a 2012 analogue (low emissions). We find that winter NOx reductions represent a potential new opportunity for improving air quality, due to PM2.5 being more nitrate-limited under low emissions over much of the US, particularly the Midwest. Lower NOx emissions also promote aqueous-phase SO2 oxidation, increasing the sensitivity of PM2.5 to SO 2. Sensitivities to NH3 emissions are lower in the low emissions case, primarily as a direct response to a less acidic atmosphere. Results for winter in the northern Midwest are driven by the thermodynamic behavior of ammonium nitrate aerosols and are well-constrained for the meteorological conditions (i.e., cold and moist) that prevail there, where concentrations are highest. Figure 3 shows that nitrate availability will play a major role in determining PM2.5 in this region in the near future. Accounting for the model’s high NO3− bias can only push the system further into the nitrate-limited regime. Nevertheless, the large absolute sensitivities to winter NOx emissions through NO3− formation are subject to the model bias. Several studies28,29,38 have shown that the standard GEOS-Chem simulation overestimates HNO3 and aerosol NO3− concentrations compared to both CASTNet and AQS measurements. There is evidence that certain types of NO3− measurements are biased low due to HNO3 volatilization from filters,51−53 but adjusting for this does not always provide significant improvement.54 Studies with other air quality models (notably CAMx5 and CMAQ55,56) have emphasized the potential impact of NH3 emissions controls on PM2.5 concentrations, suggesting that our results are broadly consistent across models. There are several possible sources of the nitrate bias in GEOS-Chem and other chemical transport models. The dependence of the rate of N2O5 hydrolysis on aerosol water, nitrate, chloride, and organic content is uncertain, and nitric

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ASSOCIATED CONTENT

S Supporting Information *

PM2.5 component concentration maps, a detailed modelmeasurement comparison, further analysis of the SO2 oxidation 4839

DOI: 10.1021/acs.est.5b00008 Environ. Sci. Technol. 2015, 49, 4834−4841

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pathways, and a comparison of a linear extrapolation to the full model. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 617 253 6281. Fax: 617 258 7733. E-mail: jareth@mit. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with support from the MIT Energy Initiative Seed Fund program. REFERENCES

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Policy Analysis

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DOI: 10.1021/acs.est.5b00008 Environ. Sci. Technol. 2015, 49, 4834−4841